Glass sheet suitable to toughening and toughened glass using said glass

ABSTRACT

The present invention provides a glass sheet having both temperability and shapability. This glass sheet has a soda-lime silica composition including 13.5 wt % to 16.0 wt % of Na 2 O. When the glass sheet is heated and then is quenched rapidly to become a tempered glass, the tempered glass satisfies a formula of T2−T1≧5, where T1 denotes a minimum heat-treatment temperature that is required for obtaining a tempered glass with a minimum number of fragments of at least 50, which is determined based on the method of fragmentation test for uniformly tempered glass according to ECE R43, while T2 denotes a maximum heat-treatment temperature that allows a tempered glass to be obtained, with the tempered glass permitting an angle of 25° or smaller to be formed between the normal orientation of a screen surface and the orientation of the surface of a glass sheet having a maximum optical distortion of not higher than 2.4 minutes in a test area D that is measured according to the optical distortion test prescribed in JIS R3212.

TECHNICAL FIELD

The present invention relates to a glass sheet that has an excellentbalance of shapability and temperability and is suitable for beingtempered by air cooling. The present invention also relates to temperedglass produced using the glass sheet.

BACKGROUND ART

JP61(1986)-197444A and JP62(1987)-246839A propose soda-lime silica glasssheets that are subjected easily to compressive stress by rapidquenching, i.e. that have excellent temperability.

JP61-197444A discloses a glass sheet having a composition consistingessentially of, in % by weight, 63% to 75% of SiO₂, 1.5% to 7% of Al₂O₃,0% to 6% of TiO₂, 5% to 15% of CaO, 0% to 10% of MgO, 8% to 18% of Na₂O,and 0% to 5% of K₂O, wherein the sum of TiO₂ and Al₂O₃ is 3% to 7%, thesum of MgO and CaO is 6% to 20%, and the sum of Na₂O and K₂O is 10% to20%. This composition cannot avoid containing a fair amount of Al₂O₃ inorder to avoid unwanted coloring caused by TiO₂. An increased amount ofAl₂O₃ results in a composition that is difficult to melt.

JP62-246839A discloses a glass sheet having a composition including, in% by weight, 68.0% to 71.0% of SiO₂, 1.6% to 3.0% of Al₂O₃, 8.5% to11.0% of CaO, 2.0% to 4.0% of MgO, 12.5% to 16.0% of Na₂O, and 0.9% to3.0% of K₂O so that the sum thereof is at least 97%, wherein the sum ofSiO₂ and Al₂O₃ is 70.0% to 73.0%, the sum of CaO and MgO is 12.0% to15.0%, and the sum of Na₂O and K₂O is 13.5% to 17.0%. In thiscomposition, the component ratio is adjusted so that the temperature atwhich the viscosity reaches 10⁹ poises is 650° C. to 680° C., thetemperature at which the viscosity reaches 10¹² poises is 555° C. to585° C., and the difference therebetween is 96° C. to 103° C.

In the compositions disclosed in JP61-197444A and JP62-246839A, theFe₂O₃ content is limited. However, glass compositions in which Fe₂O₃content is increased to improve the infrared absorptivity also have beenknown. For instance, WO94/14716 discloses a glass composition thatincludes, in % by weight, 69% to 75% of SiO₂, 0% to 3% of Al₂O₃, 2% to10% of CaO, 0% to 2% of MgO, 9% to 17% of Na₂O, 0% to 8% of K₂O, and0.2% to 1.5% of Fe₂O₃ (total iron oxide), and further may contain acomponent such as fluorine, wherein the total amount of alkaline-earthoxides is 10% or less. WO94/14716 describes that for desired lighttransmittance, it is important to limit the sum of MgO, CaO, and BaO to10 wt % or less. The glass composition disclosed in WO94/14716 allows atransmittance of less than 30% in the infrared region to be obtained ina 3.85-mm thick glass sheet.

DISCLOSURE OF THE INVENTION

When intended for use as window glass for vehicles such as automobiles,a flat glass sheet often is formed into a desired shape while beingtempered in a heating process. If the resultant tempered glass hasgreater optical distortion caused through its formation, it loses itscommercial value as window glass. The optical distortion that is causedthrough heating and rapid quenching depends considerably on the ease ofchanging the sheet shape, i.e. its shapability. The shapability has notgained attention until now but is a very important property togetherwith temperability, particularly in tempering a thin glass sheet.

An object of the present invention is to provide a glass sheet that hasboth temperability and shapability in practical ranges and has acomposition suitable particularly for manufacturing a thin temperedglass. The present inventors succeeded in manufacturing a glass sheetthat has a soda-lime silica composition including 13.5 wt % to 16.0 wt %of Na₂O through some experiments, wherein when the glass sheet istempered by air cooling to be a tempered glass, the tempered glasssatisfies the following formula:T2−T1≧5.

In the above formula, T1 denotes a minimum heat-treatment temperature (°C.) that is required for obtaining a tempered glass with a minimumnumber of fragments of at least 50, which is determined based on themethod of fragmentation test for uniformly tempered glass according toECE R43 (Economic Commission for Europe Regulation No. 43; Agreementconcerning the adoption of uniform technical prescriptions for wheeledvehicles, equipment and parts and the conditions for reciprocalrecognition of approvals granted on the basis of these prescriptions).The higher the minimum number of fragments, the higher the tensilestress existing inside the tempered glass.

On the other hand, T2 denotes a maximum heat-treatment temperature (°C.) that allows a tempered glass to be obtained, with the tempered glasspermitting an angle of 25° or smaller to be formed between the normalorientation of a screen surface and the orientation of the surface of aglass sheet having a maximum optical distortion of not higher than 2.4minutes in a test area D that is measured according to the opticaldistortion test prescribed in Japanese Industrial Standard (JIS) R3212.The smaller the angle (mounting angle), the smaller the opticaldistortion of the tempered glass.

A lower T1 and a higher T2 denote that the glass sheet has excellenttemperability and excellent shapability, respectively. T2 should behigher than T1, and a larger value of T2−T1 is preferable, whenconsideration is given to mass production.

In the present invention, the thickness of the glass sheet is notlimited. However, when being applied to a glass sheet that is relativelythin, for instance, thinner than 2.6 mm and particularly thinner than2.4 mm, the present invention provides a great effect in manufacturingtempered glass.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the unit “%” indicating an amount of contained alwaysdenotes “wt %”.

In the present invention, the glass composition includes 13.5% to 16.0%,preferably 14.0% to 15.0% of Na₂O. Na₂O is a component that reduces theviscosity of a glass material to improve productivity. In addition, Na₂Oincreases the thermal expansion coefficient of the glass to improve itstemperability. It is important to adjust the content of Na₂O within theabove-mentioned range in order to obtain both temperability andshapability.

In a preferable embodiment of the present invention, the glasscomposition includes 0.4% to 2% of total iron oxide (hereinafterreferred to as “T-Fe₂O₃”) in terms of Fe₂O₃. Iron oxide exists in theform of Fe₂O₃ and FeO in the glass. Fe₂O₃ and FeO improve theultraviolet absorptivity and infrared (heat ray) absorptivity,respectively. In order to obtain these absorptivities suitably, it ispreferable that the content of T-Fe₂O₃ is at least 0.4%, particularly atleast 0.5% while the ratio of FeO in terms of Fe₂O₃ to T-Fe₂O₃(hereinafter referred to as a “FeO ratio”) is 15% to 50%, particularly20% to 30%. When the content of T-Fe₂O₃ exceeds 2%, the visible lighttransmittance decreases. When the FeO ratio exceeds 50%, the glassmaterial becomes highly reducible in manufacturing processes to generatesilica scum and thereby productivity decreases.

Generally, in a glass sheet with high heat ray absorptivity, itstemperature not only rises easily but also decreases easily. Hence, in aprocess of tempering a glass sheet, the temperature of the glass sheetdecreases considerably during the period from the time it is taken outof a heating furnace to the time compressed air is blown on it.Especially, the heat capacity of the glass sheet decreases with thereduction in its thickness. Accordingly, this rapid temperature decreasetends to cause a problem in tempering a thin glass sheet. In the case ofa composition including a relatively large amount of T-Fe₂O₃ to improvethe heat ray absorptivity, it is desirable to deal with the decrease intemperature of the glass sheet by suppressing radiation through theadjustment of the contents of other components.

According to a preferable embodiment of the present invention, even whenthe content of T-Fe₂O₃ is 0.4% to 2% and that of FeO content is 15% to50%, it is possible to obtain a composition with a temperaturedecreasing rate of 8.0° C./s or lower. In the present specification, the“temperature decreasing rate” denotes a mean temperature decreasing rateof a glass sheet obtained when the glass sheet is kept in air at 25° C.for 10 seconds, with the glass sheet having a thickness of 2.2 mm andhaving been heated to 600° C. Although not all the factors that affectthe temperature decreasing rate have been figured out, at least it iscertain that 13.5% to 16.0% of Na₂O has an effect on reduction in thetemperature decreasing rate.

Preferably, the glass sheet of the present invention has a compositionincluding: 65% to 80% of SiO₂; 0% to 5% of Al₂O₃; 0% to 10% of MgO; atleast 5% but less than 8.5% of CaO; 13.5% to 16.0% of Na₂O; 0% to 5% ofK₂O; 0% to 5% of B₂O₃; 0.4% to 2% of T-Fe₂O₃; 0% to 1% of TiO₂; and 0%to 2% of CeO₂, wherein the sum of MgO and CaO is more than 10% but 15%or less, the sum of Na₂O and K₂O is less than 20%, the sum of Al₂O₃ andTiO₂ is less than 3%, and the FeO ratio is 15% to 50%. From anotheraspect, the present invention provides a tempered glass having thepreferable composition described above and a thickness of less than 2.6mm, particularly less than 2.4 mm, for example, at least 1.0 mm but lessthan 2.6 mm.

A preferable embodiment of the present invention provides a glasscomposition that allows the temperature difference between its softeningpoint and its strain point to be 230° C. or lower. This temperaturedifference is used, for convenience, to serve as an index for indicatingthe inclination of a viscosity curve. In the present specification, tobe precise, the softening point is defined as a temperature at which anequation of log η=7.6 holds and the strain point as a temperature atwhich an equation of log η=14.5 holds. In the equations, η denotesviscosity of glass. In the temperature region between the softeningpoint and the strain point, deformation may be caused due to viscousflow. In order for a shaped glass sheet to maintain its shape and alsoto be tempered, it is preferable that the glass sheet includesapproximately an above-mentioned amount of Na₂O and the temperaturedifference is smaller.

A soda-lime silica glass composition includes Na₂O, CaO and SiO₂, whichare essential components, as well as other optional components such asthose exemplified above. The content of such components may be selectedform a wider range than that exemplified above as long as the object ofthe present invention is achieved. The respective components other thanNa₂O and Fe₂O₃ are described below.

SiO₂ is a main component forming a glass network. If the content of SiO₂is less than 65%, the glass has reduced durability. On the other hand,if the content of SiO₂ exceeds 80%, the glass is difficult to melt.Preferably, the content of SiO₂ is 65% to 80%. Moreover, in order toobtain both temperability and shapability, it is preferable that thecontent of SiO₂ is 67% to 71%.

Al₂O₃ is a component that improves durability of glass. If the contentof Al₂O₃ exceeds 5%, the glass is difficult to melt. Preferably, thecontent of Al₂O₃ is 5% or lower. Moreover, in order to obtain bothtemperability and shapability, it is preferable that the content ofAl₂O₃ is 1.1% to 2.0%, particularly 1.4% to 1.7%.

MgO and CaO both improve durability of glass. The content of thesecomponents may be adjusted to control the viscosity and devitrificationtemperature of the glass during formation thereof. If the content of MgOexceeds 10% or the content of CaO is less than 5% or exceeds 15%, ahigher devitrification temperature results. If the sum of MgO and CaO isless than 5%, the glass has reduced durability. On the other hand, ifthe sum thereof exceeds 15%, a higher devitrification temperatureresults. A preferable content of CaO is 5% to 15%. In order to obtainboth temperability and shapability, it is preferable that the content ofCaO is at least 5% but lower than 8.5%, particularly 7.0% to 8.4%. Apreferable content of MgO is 10% or lower. In order to obtain bothtemperability and shapability, it is preferable that the content of MgOis 2.5% to 4.5%. Preferably, the sum of MgO and CaO is 5% to 15%. Inorder to obtain both temperability and shapability, it is preferablethat the sum thereof is higher than 10% but 15% or less, particularly11% to 14%.

K₂O promotes melting of glass and is considered to contribute to theimprovement in temperability and shapability together with Na₂O.However, a large amount of K₂O results in high manufacturing cost.Accordingly, it is preferable that the content of K₂O is 5% or lower,particularly 1% or lower. When the sum of Na₂O and K₂O exceeds 20%, theglass has reduced durability.

B₂O₃ is used for improving durability of glass or as a melting aid andalso has an effect of improving ultraviolet absorption. A content ofB₂O₃ exceeding 5% causes inconvenience due to its volatilization or thelike in forming the glass. If B₂O₃ is included, its content ispreferably 5% or lower. There is no problem even if substantially noB₂O₃ is included.

TiO₂ improves ultraviolet absorptivity of glass through interaction withFeO. When this effect is intended to be obtained, it is suitable to addat least 0.01% of TiO₂. When the content of TiO₂ exceeds 1%, themanufacturing cost increases and the glass tends to be yellowish. TiO₂is a component that contributes to the improvement in temperabilitytogether with Al₂O₃. According to the present invention, however, boththe temperability and shapability can be obtained even when the sum ofAl₂O₃ and TiO₂ is controlled to less than 3% to eliminate undesirablecoloring and to keep the meltability of the glass at the same time.

CeO₂ is a component that improves ultraviolet absorptivity of glassthrough interaction with Fe³⁺. Cerium exists as Ce³⁺ or Ce⁴⁺ in glass.The Ce³⁺ absorbs less light in the visible region and therefore iseffective for ultraviolet absorption. When the ultraviolet absorption isintended to occur, a suitable content of CeO₂ (the amount of totalcerium oxide in terms of CeO₂) is at least 0.01%. The content of CeO₂exceeding 2% results in higher production cost and lower visible lighttransmittance.

When the glass composition includes TiO₂ and/or CeO₂ (TiO₂+CeO₂>0%), thecontent of T-Fe₂O₃ may be controlled to 1% or less.

Sulfate of alkali or alkaline-earth metal may be used as a refiningagent in the glass material. In this case, the glass composition mayinclude 0.5% or less, for instance, 0.1% to 0.5% of SO₃. Furthermore,the glass composition also may include 1% or less of the sum of Sb₂O₃,SnO₂, and the like that are added as a reducing agent or refining agentthereto. Moreover, 1% or less of ZnO may be included in the glasscomposition to prevent nickel sulfide from being generated.

Impurities other than those described above, for instance, Se, CoO,Cr₂O₃, Mn₂O₃, CuO, Nd₂O₃, Er₂O₃, MoO₃, V₂O₅, La₂O₃, or NiO may beintroduced into the glass due to the use of cullets in the raw material.These impurities may be included in the glass, preferably in the rangeof total amount of 0.1% or less, as long as the object of the presentinvention is achieved.

EXAMPLES Example 1

Suitable amounts of ferric oxide, titanium oxide, cerium oxide, and acarbon-based reducing agent (carbon powder, etc.) were mixed with batchcomponents of typical soda-lime silica glass. This raw material wasmelted using a common melting furnace for soda-lime silica glass andthen a 2.2-mm thick glass sheet was produced by the float glass process.This glass sheet was cut. Thus a glass sheet with a size of about 300mm×about 300 mm was obtained.

This glass sheet was hung with tongs to be carried into an electricfurnace and then was heated to a predetermined temperature. Thereafter,it was carried out of the electric furnace. Subsequently, using a groupof nozzles with a diameter of 4 mm that were arranged at a pitch of 30mm×40 mm, compressed air having room temperature was blown on both thewhole surfaces of the glass sheet. Thus the glass sheet was tempered byair cooling. The pressure of the compressed air was set at 8000 mmAq.The temperature (heating temperature) of the glass sheet that had beenheated was measured with a pyrometer right after the glass sheet wascarried out of the electric furnace and before the compressed air wasblown on it.

The tempered glass sheet thus obtained was subjected to a determinationof the minimum number of fragments of the glass according to the methodof fragmentation test for uniformly tempered glass that is described inECE R43. The minimum number of fragments was determined by the number offragments included in the area of 5 cm×5 cm having the lowest number offragments in the shattered glass sheet. Furthermore, the angle (themounting angle) between the normal orientation of a screen surface andthe orientation of the surface of a glass sheet was measured, with theglass sheet having a maximum optical distortion of not higher than 2.4minutes in the test area D that was measured according to the opticaldistortion test defined in JIS R3212. The minimum temperature T1 atwhich the minimum number of fragments was at least 50 then wasdetermined from the relationship between the minimum number of fragmentsand the heating temperature of the glass sheet. In addition, the maximumtemperature T2 that allows the mounting angle to be 25° or smaller wasdetermined from the relationship between the mounting angle and theheating temperature of the glass sheet.

Furthermore, with respect to the glass prepared from the same rawmaterial as that described above, the temperature (softening point) atwhich its viscosity η satisfied the condition of log η=7.6 and thetemperature (strain point) at which its viscosity η satisfied thecondition of logη=14.5 were determined. The softening point and thestrain point were measured by the penetration test for viscosity and thebeam bending method (ASTM C-598), respectively.

Furthermore, with respect to a 2.2-mm thick glass sheet that wasobtained in the same manner as in the above but had not been temperedyet, its temperature was measured with a pyrometer after the glass sheetheated to 600° C. using the electric furnace was taken out of theelectric furnace and was maintained at room temperature (25° C.) for 10seconds. The mean temperature decreasing rate (° C./s) was determinedfrom the temperature decrease caused in 10 seconds from theabove-mentioned temperature measured with the pyrometer. In this case,the temperature of the center of the glass sheet was measured and a meanvalue of results of the measurements carried out three times wasemployed. The measurement results are shown in Table 1 together with theglass composition.

Examples 2 to 3 and Comparative Examples 1 to 3

Tempered glass was manufactured in the same manner as in Example 1except that the glass composition was changed, and its characteristicswere determined. Table 1 shows the glass composition and the determinedcharacteristics.

In the respective examples, the difference between T2 and T1 was atleast 5° C., the difference between the softening point and the strainpoint was 230° C. or lower, and the temperature decreasing rate was nothigher than 8.0° C./s. Particularly, in Example 1, the differencebetween T2 and T1 exceeded 7° C., the difference between the softeningpoint and the strain point was smaller than 220° C., and the temperaturedecreasing rate was lower than 8.0° C. /s. The compositions described inthe respective examples also are suitable for recycling the glass sheetas cullets and therefore also are excellent from the viewpoint ofresource recycling. TABLE 1 Examples Comparative Examples 1 2 3 1 2 3Glass Na₂O 14.31 13.51 13.63 12.88 13.20 12.74 Compo- T-Fe₂O₃ 0.88 0.511.25 0.53 0.48 0.64 sition FeO/T-Fe₂O₃ 28.9 29.1 24.9 26.7 26.9 37.9 wt% SiO₂ 70.1 71.1 71.0 71.3 70.9 70.8 Al₂O₃ 1.53 1.54 1.62 2.05 1.88 1.80CaO 8.21 8.16 7.71 8.16 8.18 7.60 MgO 3.12 4.09 3.60 3.67 4.24 3.72 K₂O0.64 0.83 0.85 1.02 0.74 0.61 SO₃ 0.17 0.20 0.17 0.19 0.17 0.07 TiO₂0.05 0.02 0.02 0.04 0.07 0.36 CeO₂ 0.92 0.02 0.04 0.13 0.10 1.58 CoO0.0095 NiO 0.0294 T2 (° C.) 593 597 590 602 602 608 T1 (° C.) 585 591584 600 600 607 T2-T1 (° C.) 8 6 6 2 2 1 Softening Point (° C.) 719 728725 737 736 736 Strain Point (° C.) 502 503 503 501 498 500 SofteningPoint-Strain 217 225 222 236 238 236 Point (° C.) Temperature Decreasing7.7 8.0 8.0 8.1 8.1 8.3 Rate (° C./s)

1. A glass sheet, having a soda-lime silica composition comprising 13.5wt % to 16.0 wt % of Na₂O, wherein when the glass sheet is heated andthen is quenched rapidly to become a tempered glass, the tempered glasssatisfies a formula of T2−T1≧5, where T1 denotes a minimumheat-treatment temperature that is required for obtaining a temperedglass with a minimum number of fragments of at least 50, which isdetermined based on the method of fragmentation test for uniformlytempered glass according to ECE R43, while T2 denotes a maximumheat-treatment temperature that allows a tempered glass to be obtained,with the tempered glass permitting an angle of 25° or smaller to beformed between a normal orientation of a screen surface and anorientation of a surface of a glass sheet having a maximum opticaldistortion of not higher than 2.4 minutes in a test area D that ismeasured according to the optical distortion test prescribed in JISR3212.
 2. The glass sheet according to claim 1, wherein the compositioncomprises 0.4 wt % to 2 wt % of total iron oxide in terms of Fe₂O₃, andin the composition, FeO in terms of Fe₂O₃ accounts for 15 wt % to 50 wt% of the total iron oxide.
 3. The glass sheet according to claim 2,wherein the composition has a temperature decreasing rate of 8.0° C./sor lower, where the temperature decreasing rate denotes a meantemperature decreasing rate of a glass sheet obtained when the glasssheet is kept in air at 25° C. for 10 seconds, with the glass sheethaving a thickness of 2.2 mm and having been heated to 600° C.
 4. Theglass sheet according to claim 1, wherein in the composition, atemperature difference between its softening point and its strain pointis 230° C. or lower.
 5. The glass sheet according to claim 1, whereinthe composition comprises 14.0 wt % to 15.0 wt % of Na₂O.
 6. The glasssheet according to claim 1, having a composition comprising, in % byweight: 65% to 80% of SiO₂; 0% to 5% of Al₂O₃; 0% to 10% of MgO; atleast 5% but less than 8.5% of CaO; 13.5% to 16.0% of Na₂O; 0% to 5% ofK₂O; 0% to 5% of B₂O₃; 0.4% to 2% of total iron oxide in terms of Fe₂O₃;0% to 1% of TiO₂; and 0% to 2% of CeO₂, wherein the sum of MgO and CaOis more than 10% but 15% or less, the sum of Na₂O and K₂O is less than20%, the sum of Al_(2l O) ₃ and TiO₂ is less than 3%, and FeO in termsof Fe₂O₃ accounts for 15 wt % to 50 wt % of the total iron oxide.
 7. Atempered glass obtained by heating a glass sheet according to claim 1and then rapidly quenching it.
 8. The tempered glass according to claim7, wherein the tempered glass has a thickness of less than 2.6 mm.
 9. Atempered glass, having a thickness of less than 2.6 mm and a compositioncomprising, in % by weight: 65% to 80% of SiO₂; 0% to 5% of Al₂O₃; 0% to10% of MgO; at least 5% but less than 8.5% of CaO; 13.5% to 16.0% ofNa₂O; 0% to 5% of K₂O; 0% to 5% of B₂O₃; 0.4% to 2% of total iron oxidein terms of Fe₂O₃; 0% to 1% of TiO₂; and 0% to 2% of CeO₂, wherein thesum of MgO and CaO is more than 10% but is 15% or less, the sum of Na₂Oand K₂O is less than 20%, the sum of Al₂O₃ and TiO₂ is less than 3%, andFeO in terms of Fe₂O₃ accounts for 15 wt % to 50 wt % of the total ironoxide.
 10. The glass sheet according to claim 9, wherein the compositioncomprises 14.0 wt % to 15.0 wt % of Na₂O.